A polarization multiplexing technology which doubles amount of information per wave is put into practical use by putting different information, respectively on two polarized waves orthogonal to each other in an optical fiber. For example, the polarization multiplexing technology is disclosed in non-patent documents 1 and 2.
When the polarization multiplexing technology is applied, it is necessary to make each polarized wave of an optical signal the same intensity. However, because a PDL (Polarization Dependent Loss) exists in an optical component of which an optical network system is composed, an intensity mismatch is generated in response to a different loss for every polarized optical signal.
Accordingly, various technologies which reduce the optical intensity mismatch between the polarized optical signals caused by the PDL are proposed. For example, a polarization multiplexing optical receiver is disclosed in patent document 1, in which a polarization multiplexing optical signal is entered into a polarization beam splitter via a polarized wave controller for controlling a polarized state of an optical signal, and spectrally separated it into two polarization optical signals orthogonal to each other using the polarization beam splitter and outputted. The polarization multiplexing optical receiver reduces a mismatch of optical intensity between the polarized optical signals in the optical transmission path, by performing feedback control of the polarized wave controller so that the sum of the intensity of each polarized optical signal may become the maximum.
On the other hand, a polarized wave diversity receiver which compensates a difference in input distribution between the first polarized optical signal and the second polarized optical signal is disclosed in patent document 2, although it is not technology for reducing a mismatch of optical intensity between the polarized optical signals caused by PDL. A block diagram of a polarized wave diversity receiver of patent document 2 is shown in FIG. 4.
In FIG. 4, a polarized wave diversity receiver 900 performs electric conversion of the polarized wave mixed light outputted from an optical mixer 910 by using optical detector circuits 921, 922, and monitors the level of each electric current by using monitors 931, 932. Then, a polarized wave distribution difference ratio detection 940 and a distribution difference compensation circuit 950 control the gain of variable gain amplifier circuits 961, 962 using the monitored result.
The variable gain amplifier circuits 961, 962 amplify the first and second polarized wave mixed lights so that the levels of each electric current may become same, so the difference in the input distribution between the first and the second polarized wave mixed lights is compensated by a feed-forward form.